This invention relates to a magnetic field coil forming an at least approximately uniform magnetic field in the space in a cylindrical coil, and more particularly to a superconducting magnet for a nuclear magnetic resonance imaging apparatus providing a high magnetic field (hereinafter referred to as "an MRI apparatus", when applicable).
A superconducting magnet for an MRI apparatus comprises: a cylindrical coil or a plurality of ring-shaped coils about 1 m in diameter for receiving therein the human body to be examined; and, when necessary, a magnetic shield made of magnetic material such as steel which is arranged outside the coils and absorbs the leakage flux of the coils. In order to obtain a tomogram of the human body, it is required that the strength of the magnetic field in the coils in which the human body is laid is uniform with a tolerance of 1/1,000,000 or less. In order to obtain the strength of the magnetic field and the uniformity in strength, heretofore the following method is employed: A uniform magnetic field coil is formed by coaxially arranging a plurality of ring-shaped coils so that, when current flows therein, the magnetic field formed thereby is most uniform in strength.
However, the uniform magnetic field coil thus formed is disadvantageous in that the uniform magnetic field formed thereby leaks out. That is, the leakage magnetic field causes electronic equipment around the magnet to operate erroneously, and distorts the magnetic field of a ferromagnetic element near the MRI apparatus, whereby the uniform magnetic field is adversely affected. Thus, although the uniform magnetic field coil itself can form the uniform magnetic field, when it is singly used with the MRI apparatus the magnetic field uniformity is not so high as expected.
In order to eliminate the above-described difficulty attributing to the leakage magnetic field, a method has been employed in which a magnetic shield is provided outside the uniform magnetic field coil.
The magnetic shield is shown in FIG. 7. The magnetic shield comprises a cylinder made of magnetic material such as steel, and it is provided outside a cryostat 20A. The cryostat 20A accommodates a main coil 3A comprising superconducting coils. The magnetic shield is referred to as "a self-shield". The self-shield absorbs the leakage magnetic field of the main coil 3A. This method is applied not only to a superconducting magnet but also an MRI magnet using conducting coils. The method suffers from the following difficulty: Since in the case of an MRI apparatus providing a high uniform magnetic field strength, the leakage magnetic field is large, the self-shield made of steel is considerably large in weight, so that the room where the MRI apparatus is installed may be insufficient in mechanical strength.
In order to eliminate the above-described difficulty accompanying the employment of a magnetic shield of ferromagnetic material for a superconducting magnet, a method has been employed in which, as shown in FIG. 8, the leakage magnetic flux is cancelled with a superconducting coil which is the same as the main coil. In FIG. 8, reference character 3B designates the main coil; and 2B, the superconducting coil which is accommodated in the same cryostat 20B as the main coil 3B. The superconducting coil 2B is referred to as "an active shield".
When the magnetic moment of the active shield 2B is made equal in absolute value to and opposite in direction to that of the main coil, then the leakage magnetic field can be ideally cancelled out (cf. Japanese Patent Application (OPI) No. 217608/1985 (the term "OPI" means an "unexamined published application")). On the other hand, the magnetic flux density of the uniform magnetic field space induced by the active shield 2B is opposite in direction to the magnetic flux density of the uniform magnetic field induced by the main coil 3B, and therefore the magnetic flux density of the uniform magnetic field which is the composite of these two magnetic flux densities is smaller than that of the uniform magnetic field provided when only the main coil is employed. In order to complement the reduction of the magnetic field density, it is necessary to increase the ampere turn of the main coil 3, and accordingly the quantity in use of superconducting wire is increased not only for the active shield 2B but also for the main coil 3B.
The above-described method of using as the magnetic shield the active shield comprising the superconducting coil is advantageous in the following point: The leakage magnetic field can be cancelled out without use of a magnetic material, and therefore the uniform magnetic field space of the magnetic material near the MRI apparatus is scarcely affected, with the result that a magnetic field analysis necessary for designing a uniform magnetic field coil can be readily achieved. Thus, the method is effective in designing a superconducting magnet for the MRI apparatus in which the magnetic field space must be considerably high in uniformity. The above-described superconducting MRI apparatus magnet in which the leakage magnetic field is cancelled with the same superconducting coil as the uniform magnetic field coil is referred to as "an active shield type superconducting MRI apparatus magnet". The above-described system is advantageous in that, since no magnet material such as steel is employed, the weight is light, and installation of the MRI apparatus is free from the load limit of the floor. However, it is still disadvantageous in the following points: When compared with the self-shield type superconducting magnet as shown in FIG. 7, the quantity of superconducting wire used for formation of the superconducting coil is large. Since superconducting wire is much higher in cost than steel, the superconducting magnet is higher in manufacturing cost than the self-shield type superconducting magnet, although the former is smaller in size than the latter.
If, in the case when a superconducting MRI apparatus is installed in the room, the total weight of the superconducting magnet determined from the load limit of the floor of the room is excessively large because of the employment of the self-shield as the magnetic shield, then it is necessary to employ the active shield. However, in this case, the superconducting magnet high in manufacturing cost must be used, and the MRI apparatus high in manufacturing cost must be employed.